The sensor inside the D600 will probably be 24MP (made by Sony, modified by Nikon)

The new camera will be marketed as an entry level full frame camera

Auto DX crop mode

In-camera RAW editor

Built in time-lapse functionality

Possibly with build-in HDR

Possibly with integrated GPS

New external battery grip

To be released this summer

The D600 will probably not have an internal AF motor, which means it will work only with AF-S lenses (just like the D3200 and D5100)

The price of the D600 is rumored to be very low - maybe as low as $1500

Announcement before Photokina (September 2012)

One or more low-priced f/4 lenses will be announced with the D600. For example, Nikon recently filed a patent for a 24-70mm f/3.5-4.5 full frame lens which seems to be designed for a cheaper FX DSLR body

At that point I feel comfortable to raise the [NR] probability for the Nikon D600 to 60%. Some of the listed specs may still be inaccurate, but I think I got the most part correct.